Thermosetting powder coatings have gained considerable popularity in recent years over liquid coatings for a number of reasons. Powder coatings are virtually free of harmful fugitive organic solvents normally present in liquid coatings, and, as a result, give off little, if any, volatiles to the environment when cured. This eliminates solvent emission problems and dangers to the health of workers employed in coating operations. Powder coatings also improve working hygiene, since they are in dry solid form and have no messy liquids associated with them to adhere to workers' clothes and coating equipment. Furthermore, they are easily swept up in the event of a spill without requiring special cleaning and spill containment supplies. Another advantage is that they are 100% recyclable. Over sprayed powders are normally recycled during the coating operation and recombined with the original powder feed. This leads to very high coating efficiencies and minimal waste generation.
Despite many advantages, powder coatings traditionally have not been used for coating heat sensitive substrates, such as wood and plastic articles, due to the rather high temperatures demanded for flow and cure. Recently, the powder coating industry has concentrated its efforts on developing low temperature curable powders. These new generation powders permit polymerization or curing at much lower temperatures, reducing the potentially damaging and deforming heat loads imposed on sensitive substrates.
One class of low temperature curable powder recently developed are the UV curable powders. UV curable powders have the ability to flow and cure and produce smoother coatings at much lower temperatures than previously possible with traditional thermosetting chemistry. This is primarily due to the curing reaction being triggered by photoinitiated radiation rather than heat. Typically, UV powders are formulated from solid unsaturated base resins with low Tg, such as unsaturated polyesters, unsaturated co-polymerizable crosslinker resins, such as vinyl ethers, photoinitiators, flow and leveling agents, performance-enhancing additives, and, if necessary, pigments and fillers. It is also common to replace all or part of the base resins or crosslinkers with crystalline materials to provide powders with lower melt viscosity and better flow out behavior.
During coating operations, UV curable powders are applied to a substrate in the usual fashion, using electrostatic spray techniques. The coated substrate is then heated for as long as it takes to drive out substrate volatiles and fuse the powders into a smooth molten coating. Immediately following fusion, the molten coating is exposed to UV light, which, in an instant, cures and hardens the film into a durable, extraordinarily smooth, attractive coating.
One drawback of UV curable powders is that it is very hard to produce a low gloss (i.e., matte) coating. The coatings formed tend to have a relatively high glossy appearance. For reasons of aesthetic preference, it would be desirable to have UV curable powder coatings which provide low gloss coatings. Gloss reduction can normally be obtained in traditional powder coatings through the introduction of matting agents, such as fillers or waxes, which rise to the surface during curing and cause matting through disruption of the surface of the coating. However, because UV curable powders cure so quickly, there is not adequate time for the fillers and waxes to flocculate to the surface, and they become trapped within the coating. There is reduction in flow in the coating but little matting takes place. Higher amounts of filler or waxes may be used, but this tends to cause the powders to block or cake during normal storage and/or produce coatings with severe orange peel, limiting the amount of gloss reduction that could be attained.
It would be desirable to provide a method for producing cured coatings with a low gloss appearance from UV curable powders.